Fifty patients, scheduled for an elective tonsillectomy because of recurrent group A beta-hemolytic streptococcal tonsillitis, participated in a prospective randomized study that compared the efficacy of presurgical treatment with either phenoxymethyl penicillin or clindamycin hydrochloride in eradicating group A beta-hemolytic streptococci and beta-lactamase-producing bacteria (BLPB) from the tonsillar core. They were randomized into three groups as follows: 11 received penicillin, 22 received clindamycin, and 17 received no therapy. Group A beta-hemolytic streptococci were isolated from 8 (40%) of 17 untreated patients, 4 (36%) of 11 patients treated with penicillin, and none of 22 patients treated with clindamycin. Twenty-one BLPB were isolated from 16 (94%) of 17 untreated patients, 11 BLPB from 9 (82%) of 11 patients treated with penicillin, and 7 BLPB from 7 (32%) of 22 patients treated with clindamycin compared with penicillin or no therapy. Of the 22 patients treated with clindamycin, 10 were younger than 12 years of age. The BLPB were eradicated in nine patients (90%). However, BLPB were eradicated only in 6 (50%) of the 12 patients who were aged 13 years and older. These data illustrate the efficacy of clindamycin therapy in eradicating group A beta-hemolytic streptococci, as well as BLPB, in recurrent inflamed tonsils, especially in persons aged 12 years old and younger.